During adverse weather conditions visibility can be restricted by fog, rain, snow, or blowing dust. At night, the visibility restrictions may be severe even though intense lights are used. The principal reason for night time visibility restrictions is the backscattering of the light from the minute water, snow or dust particles at relatively short ranges. The result is a bright visual "curtain" which obscures the weaker reflected light from more distant objects. The objective of the present invention is to reduce the effects of this phenomenon, and to provide improvements in visual range through such diffusive media. Forwardscatter also occurs in the diffusive media, reducing the observed resolution of an object, but its effects are of much lesser magnitude. The effects of forwardscatter are not dealt with by the present invention.
U.S. Pat. No. 4,174,524 discloses a system for overcoming the effects of backscatter and forwardscatter in viewing an object through a diffusive medium. The apparatus described in the patent includes a projector for projecting successive picosecond coherent light pulses through the medium to the object to be viewed, and it also includes a light receiving element for receiving light transmitted through the medium. An optical shutter is positioned in relation to the receiving element for selectively admitting light to the receiving element. An optical path between the source and the shutter couples light of a projected pulse to open the shutter for a picosecond at a selected interval after the pulse has been projected in order to admit light of the pulse reflected from the object to the receiving means. Appropriate structure cooperates with the receiving element to provide sets of light reflective surfaces of the object which are included in one of the one or more discrete viewing spaces, and a display is provided for combining all of the represented images to provide an image of the object. The system described in U.S. Pat. No. 4,174,524 is sophisticated and expensive. In the system of the patent, the scene is not directly viewed, rather, the scene is scanned by the light pulses, and it is slowly reconstructed on the screen of a cathode-ray tube.
Unlike the system described in the patent, the system of the present invention is relatively simple and inexpensive, in that it provides for the direct viewing of the scene without scanning, and the system of the invention uses light pulses which are long as compared with the picosecond light pulses used in the system described in the patent.
U.S. Pat. No. 3,689,156 also describes a system for reducing the effects of backscatter. The system described in the latter patent provides for direct viewing through special goggles which include an electro-optic shutter, and it utilizes a continuous light source that is rapidly shuttered with an electro-optic shutter similar to the shutter used in the viewing goggles. Pochels cells are used in the shutters. These cells are limited to relatively small apertures, and they are expensive. In addition, and most important, the Pochels cells have very limited angles over which acceptable attenuation can be achieved in the opaque mode. Also, the shuttering rate of the light source in the system descrbed in the latter patent must be increased to extremely high values to approach a maximum of 50% obtainable efficiency.
The two patents referred to above are included in a class known as optical range-gated systems in which the receiver means is gated open for a relatively short distance around the object to be observed. The system of the present invention, on the other hand, does not utilize such narrow range gating, but rather provides direct viewing of the entire ambient scene with improved visibility.
A principal objective of the present invention is to provide practical visibility enhancement in diffusive media with direct viewing of a scene, the system being applicable to land, air and marine vehicles, where direct viewing is essential for safe and effective operation.
Another object of the invention is to provide a system for optimum viewing of the objects within a scene to the maximum range possible, while providing the realism of ambient lighting of the scene.
Yet another object of the invention is to provide a viewing panel in the system having electro-optic shutter means with large apertures, with operationally wide open and closed angles, and which utilizes practical operating voltages.
A still further object of the invention is to provide light flashing means within the system which are efficient and controllable so as to meet the requirements of the system.
A particular object of the invention is to provide a system having applicability to aircraft, and which includes means for improved distance viewing for the aircraft while on a final landing approach.
A further particular object of the invention is to provide means in the system for suppressing the brilliant light of headlights from on-coming vehicles.
One implementation of the system of the invention, utilizes a gaseous discharge flash lamp as its light source, the lamp being energized by short pulses at a high repetitive rate, giving the appearance of continuous illumination. The scene is directly viewed through a special panel or goggles. The panel (or goggles) operates as a high-speed optical shutter, or attenuator, which reduces its optical transmissibility by one thousand times or more for each light flash. Each of the periods of light attenuation of the scene by the panel is for the whole period of the flash plus a period after the flash cessation which is equivalent to the round-trip propagation time to a predetermined cut-off distance. For example, if the cut-off distance is 100 feet, the reflected light back through the panel would be attenuated through the period of the flash plus approximately 200 nanoseconds. Thus, the observer would see only those objects and backscatter beyond 100 feet, and the high intensity closer backscatter would be subdued, permitting viewing of the object to greater distances through fog and other diffusive media.